1. Field of the Invention
The present invention relates to a television tuner""s intermediate frequency tuning circuit, and more particularly to a television tuner""s intermediate frequency tuning circuit capable of receiving FM broadcast signals.
2. Description of the Prior Art
Referring to FIG. 8, an intermediate frequency tuning circuit of a prior art television tuner is described. A mixing circuit 21 outputs a television intermediate frequency signal or an FM broadcast signal frequency-converted to 10.7 MHz. A first filter 22 and a second filter 23 are connected to the mixing circuit 21. The first filter 22 is constituted by a parallel tuning circuit whose tuning frequency is set to the frequency of a television intermediate frequency signal (center frequency in the U.S. specifications: 43.5 MHz). The second filter 23 is constituted by a ceramic filter whose tuning frequency is set to an intermediate frequency (10.7 MHz) of common FM receivers.
When a television signal is received, a television intermediate frequency signal outputted from the mixing circuit 21 passes through the first filter 22 and is inputted to a circuit (intermediate frequency amplifier, etc.) at the next stage not shown.
When an FM broadcast signal is received, the FM broadcast signal of 10.7 MHz outputted from the mixing circuit 21 passes through the second filter 23 and is inputted to an FM demodulator or the like not shown.
As has been described above, an intermediate frequency tuning circuit of a prior art television tuner has been large in size and expensive because it has two filters, of which one is the second filter 23 for FM intermediate frequency signals that is constituted by a ceramic filter.
Accordingly, the present invention provides a small-size and inexpensive television tuner""s intermediate frequency tuning circuit that can be used for both television intermediate frequency signals and FM broadcast signals frequency-converted to 10.7 MHz.
To solve the above-described problems, the present invention provides a television tuner""s intermediate frequency tuning circuit, comprising: a parallel tuning circuit, consisting of a first inductance element and a first capacitive element, to both ends of which a television intermediate frequency signal or an FM broadcast signal frequency-converted to 10.7 MHz is inputted, the both ends being respectively connected to both ends of a load; a series tuning circuit, consisting of a second inductance element and a second capacitive element, connected in parallel to the parallel tuning circuit; and switch means, connected in parallel to the second capacitive element, that is closed when the intermediate frequency signal is inputted, and opened when an FM broadcast signal is inputted, wherein the respective tuning frequencies of the parallel tuning circuit and the series tuning circuit are set to 10.7 MHz or more; when the switch means is closed, a parallel tuning frequency produced by the parallel tuning circuit and the series tuning circuit is set to the frequency of the intermediate frequency signal; and when the switch means is opened, the parallel tuning frequency is set to 10.7 MHz.
The first inductance element is constituted of two first small inductance elements connected in series, and a first voltage is applied to a connection point between the two first small inductance elements; the second inductance element is constituted of two split second small inductance elements, and the second capacitive element is connected between the two second small inductance elements; the switch means is constituted of first and second switch diodes the cathodes of which are connected with each other; when the intermediate frequency signal is inputted, the voltages of the cathodes are dropped below the first voltage, and the first and second switch diodes are turned on; and when the FM broadcast signal is inputted, the voltages of the cathodes are raised above the first voltage, and the first and second switch diodes are turned off.
There are provided a first varactor diode for coupling one end of the parallel tuning circuit to one end of the load, and a second varactor diode for coupling the other end of the parallel tuning circuit to the other end of the load; the anode of the first varactor diode is connected to one end of the parallel tuning circuit, and the anode of the second varactor diode is connected to the other end of the parallel tuning circuit; when the intermediate frequency signal is inputted, the voltages of the cathodes of the first and second varactor diodes are dropped below the first voltage, and the first and second varactor diodes are turned on; and when the FM broadcast signal is inputted, the voltages of the cathodes are raised above the first voltage, and the first and second varactor diodes are turned off.